[Mixed epithelial and stromal tumor of kidney: a case report].

Mixed epithelial and stromal tumor of kidney (MEST-K) is a rare benign renal tumor that was first described by Michal and Syrucek in 1998. Its frequency is 0.2-0.28% of all the renal tumors. Here, we report an additional case of MEST-K occurring in a 28-year-old woman. The patient visited a hospital with complaints of lumbago and fever caused by pyelonephritis. The computed tomography revealed hydronephrosis and a cystic tumor in the right kidney, and laparoscopic right nephrectomy was performed. The resected kidney contained a cystic lesion with a grayish-white mural nodule, in the lower portion. The entire lesion measured 5 cm in diameter, and the mural nodule 2.5 cm in diameter. Histologically, the cyst was lined with tall columnar and transitional epithelia. The mural nodule showed microcystic architectures lined with tall columnar and transitional epithelia, scattered in a compact stroma. Immunohistochemically, spindle cells in the stroma were positive for smooth muscle-specific actin, and estrogen and progesterone receptors (ER and PR). Based on these findings, the tumor was diagnosed as MEST-K. MEST-K was newly introduced to the WHO classification of renal tumors, with a pathogenesis related to long-term estrogen exposure, because of ER and PR expression in the stroma. It is important to consider the possibility of this tumor when encountering cases of cystic tumor in middle-aged and older women, and men with a previous history of estrogen administration.